This invention relates to improvements in magneto-optic media and recording and readout systems for such media to improve recording and readout performance. In particular, this invention relates to optimization of magneto-optic media for maximum signal to noise ratio (SNR) in an optimized differential readout system with enhanced recording and readout performance being further enhanced by maximum efficient use of available read/write laser powers optimized relative to designed media parameters.
In the present prior art, there are extensive teachings relating to the enhancement of Kerr magneto-optic readout effects in magneto-optic readout systems by improving properties and parameters of the recording medium or of the detection system employed for readout. These enhancements are aimed at greater Kerr rotation and contrast in the Kerr magneto-optic effect by rotation of the plane of polarization of electromagnetic radiation on reflection or transmission. A large volume of this art occurred in the decade of the 1960's and into the early 1970's. Representative examples of earlier magneto-optic readout reflective and transmissive systems can be found in U.S. Pat. Nos. 3,472,575; 3,626,114; 3,815,151 and 3,947,890. Other references include the work of D. O. Smith, representative examples of which are the articles, "Magnetic Films & Optics in Computer Memories", IEEE Transactions on Magnetics, Vol. Mag-3, No. 3, pp. 433-452 (September, 1967) and "A Multilayer Dielectric- & Magnetic-Film Memory Cell Designed For Optical Readout", Journal of Applied Physics, Volume 35-3 (Part 2), pp. 772- 773 (March 1964).
The magneto-optic work of D. O. Smith pertained primarily to magneto-optic longitudinal Kerr effect as applied to magneto-optic transducers employed for magnetic film readout of magnetic films or members having in-plane magnetization. Representative examples of this art are found in U.S. Pat. Nos. 3,196,206; 3,451,740, 3,474,428; 3,594,064 and 3,636,535.
All this art, whether for applications directed toward storage memory or readout transducers, is directed to the improvement of readout characteristics in magnetic films having the easy axis of magnetization in the plane of the magnetic film. Improvements are realized by increase of the Kerr rotation reflected from the magnetic film with an ultimate goal of enhancement of Kerr effects to an optimum value.
Studies also have been made relative to ferromagnetic and ferrimagnetic films toward Kerr effect enhancement and improved readout characteristics. Enhancement of the Kerr effect has been studied in magnetic films, e.g. MnBi, having perpendicular anisotropy. These films were coated with a dielectric transparent layer, e.g. SiO or SiO.sub.2 to enhance the Kerr effect. K. Egashira and T. Yamada, "Kerr-Effect Enhancement & Improvement Of Readout Characteristics In MnBi Film Memory", Journal of Applied Physics, Vol. 45-8, pp. 3643-3648 (August, 1974), A. Shibukawa, "Kerr Readout Characteristics of MnCuBi Thin Films", Japanese Journal of Applied Physics, Vol. 16-9, pp. 1601-1604 (September 1977), R. L. Aagard, T. C. Lee and D. Chen, "Advanced Optical Storage Techniques for Computers", Applied Optics, Vol. 11-10, pp. 2133-2139 (October, 1972) and Tu Chen, D. Cheng and G. B. Charlan, "An Investigation of Amorphous Tb-Fe Thin Films For Magneto-Optic Memory Application", IEEE Transactions on Magnetics, Mag-16, No. 5, pp. 1194-1196 (September, 1980).
The film media studied have been generally bilayer magneto-optic recording media. The readout signal-to-noise ratio (SNR), being recognized as a function of the Kerr rotation, ellipticity and reflectivity, was discussed and analyzed for improvement of readout. The dependence of the readout SNR on the thickness of the dielectric overlayer can be expressed as a function of the Kerr rotation, ellipticity and reflectivity and a determination of film thickness can be made to increase Kerr rotation and Kerr polar, transverse or longitudinal effects. Thus, the use of a light transparent overlayer or underlayer with a magneto-optic layer may be applied with optimized thickness to enhance the Kerr conversivity. In particular, see the first mentioned of the two previously identified D. O. Smith articles, page 442, U.S. Pat. No. 3,594,064 disclosing a conversion matching underlayer and the previously identified Egashira et al article employing a SiO overlayer.
The use of light transparent underlayers for enhancement of direct recording of information in an optical recording overlayer is known in the optical recording and readout arts. Referred to as trilayer recording media, information is produced in the recording layer as a physical topographical feature such as an ablation or bubble. Representative examples of this type of recording media are found in U.S. Pat. Nos. 4,270,132; 4,285,056 and 4,300,227. A thin layer of optical record-retention properties is highly absorptive at the incident light beam wavelength. The thickness of the light absorptive record layer is so related to the thickness of the light transparent underlayer that overlies a reflective surface, together with the selection of desired optical constants of the medium layers, are such that the optical reflectivity of the medium can be made minimal. In other words, an antireflective condition is established in the absorptive recording layer and a highly efficient heating of the absorptive layer can be caused by the incident beam to bring about a topographical featural change in the record layer.
While it is clear from all these teachings that light transparent overlayers or underlayers with optimized thickness may enhance the Kerr effect and the antireflection quality of the recording layer, it has not been clear how these principles and contiguous light transparent layers might be applied to magneto-optic recording media with perpendicular anisotropy to achieve enhancement of the polar Kerr conversivity and optimized SNR.
D. O. Smith, in the 1960's, recognized the importance of one or more light transparent layers above and/or beneath a light transmissive layer having magneto-optic conversivity toward the optimization of magneto-optic radiation output. He also recognized that the Kerr rotation can be made as large as desired by use of antireflection dielectric coatings to render the regular component of reflected light just about zero. He then commented, " . . . it is clear that a measurement of the resultant enhancement [Kerr rotation] does not elucidate the extent to which the basic magneto-optical interaction in the magnetic material has been enhanced, if at all, by the use of antireflection layers". [D. O. Smith, "Magnetic Films & Optics In Computer Memories", supra, page 443].